No, that's completely convoluted reasoning. Gender has nothing to do with the employer's actual selection / decision making process. The "discrimination" is based solely on the employee or applicant's status as a sex offender. The fact that most sex offenders happen to be male does not turn this into a gender discrimination matter.
Let's use some analogies. How about this: The government is always going after athletes who use steroids. Most athletes who use steroids are men. So the government is mostly going after men. Sexist! No. Gender has nothing to do it. They are just after steroid users and most of those just happen to be men.
You hit the nail on the head yourself: "amateur research" is no substitute for actual knowledge and experience with the law. So yes, discriminating against sex offenders is a legal employment practice.
My response to this question is a response to a hypothetical situation based on limited facts. I am not your attorney; you are not my client and we do not have an attorney-client relationship. If you need a lawyer, you should contact one in your area. If you would like to talk with me about your case, you can call my office.